Staphylococcus aureus is a ubiquitous Gram-positive organism implicated in a spectrum of diseases; from benign, localized skin infections to life-threatening, systemic illnesses including pneumonia, sepsis and menstrual or non-menstrual toxic shock syndromes (TSS). The virulence and pathogenicity of S. aureus is attributed to its several exotoxins. The superantigen exotoxins (SAg) are important amongst them because SAg are the most potent activators of T lymphocytes known. Certain SAg could also be used as biological weapons. Thus, SAg are important from many perspectives. Nonetheless, the molecular pathways by which SAg participate in the disease pathogenesis have not been completely understood due to lack of good animal models. The conventional mouse strains are resistant to TSS due to poor binding of SAg to mouse MHC class II. However, SAg binds more efficiently to human MHC (called HLA) class I molecules. Therefore, mice transgenically expressing HLA class II molecules readily succumb to the pathogenic effects of SAg delivered through different routes and to S. aureus infection. As the disease caused by SAg in HLA class II (HLA-DR3) transgenic mice also closely mimics the syndrome in humans, they are ideal models. Using this mouse model we have demonstrated that interferon-gamma (IFN-?) dependent small intestinal pathology plays a critical role in lethality associated with TSS. However, the molecular pathways by which INF-? contributes to lethality in TSS, either directly or indirectly through other mediators, are not clear. Therefore, it is proposed to (1) Delineate th mechanisms by which IFN-? plays a lethal role in staphylococcal SAg-induced TSS. This will be achieved by generating bone marrow chimeras between HLA-DR3.IFN-?R+/+ and HLA-DR3.IFN-?R-/- mice. Bone marrow chimeras will be challenged with SEB and several molecular pathways will be compared. However, IFN-? is also important for immunity S. aureus infections. Therefore, it is proposed to (2) Determine the role of IFN-? in the pathogenesis and immunity to MRSA-induced pneumonia and sepsis. This wil be accomplished by inducing pneumonia and sepsis in HLA-DR3.IFN-?+/+ and HLA-DR3.IFN-?R-/- mice with the MRSA isolate, USA300. Several bacteriological, immunological, biochemical and pathological parameters including mortality will be compared between these two lines of mice. It is known that HLA class II polymorphisms could strongly influence the magnitude of T cell activation and IFN-? production in response to streptococcal SAg. However, the impact of HLA-DR and HLA-DQ polymorphisms on staphylococcal SAg-driven immuneresponses has not been investigated. Therefore, it is proposed to (3) Determine the extent to which staphylococcal superantigen-induced IFN-? production is modulated by HLA class II polymorphisms thereby influencing the outcome of MRSA-induced pneumonia and sepsis. This will be investigated by a series of in vivo studies using transgenic mice expressing HLA-DR2, HLA-DR3, HLA-DR4, HLA-DQ2, HLA-DQ6 or HLA-DQ8 molecules.